1. Field of the Invention
The present invention relates to a resin sealed semiconductor device and, more specifically, to a lead frame which is used therefor and excellent in both resin fluidity and heat radiation.
2. Description of the Related Art
In a prior art semiconductor device having a number of pins or terminals, such as an IC and an LSI, wire bonding and wireless bonding are known as a means for electrically connecting a semiconductor element (chip) and external leads. According to the wire bonding, connecting electrodes such as bonding pads formed on a chip, and external leads are connected to each other by wires formed of aluminum, gold or the like. To bond the wires with the connecting electrodes or external leads, thermocompression bonding and ultrasonic bonding can be employed either alone or in combination. In the wireless bonding, all connecting electrodes formed on a chip are connected to external terminals at a time by specific bumps or metal leads. A tape carrier method, a flip chip method, a beam lead method, etc. are known as the latter bonding.
FIG. 1 is a cross-sectional view of a prior art semiconductor device constituted by wire bonding. The semiconductor device is protected by a resin sealing member 2 and specifically it is constituted by mounting a semiconductor element (chip) 1 on a lead frame 3, electrically connecting them by a bonding wire, and sealing them with resin. The lead frame 3 is made of metal such as an alloy of Fe and 42Ni and formed integrally with a die pad 5, inner leads 7 and outer leads 6 as one component.
The semiconductor chip 1 is adhered onto the die pad 5 by a conductive adhesive or the like. The size of the die pad is usually the same as that of the chip 1. The ends of the inner leads 7 are opposed to the chip 1. The chip 1 and inner leads 7 are connected to each other using bonding wires 4. Each of the bonding wires is connected to its corresponding connecting electrode (not shown) formed on the major surface of the chip 1. The chip 1, bonding wires 4, die pad 5 and inner leads 7 are sealed with the resin sealing member 2 such as epoxy resin.
In the recent semiconductor device, the amount of heat generating from a chip is increased in accordance with a high degree of integration and a high-speed operation. The heat-generating chip necessitates effectively radiating the heat. To do so, TAB (Tape Automated Bonding) can be adopted and, specifically, a tape carrier (TAB tape) can be used in place of bonding wires to electrically connect the chip and lead frame, thereby increasing the size of a die pad.
FIG. 2 is a cross-sectional view of a resin sealed semiconductor device using a TAB tape. This semiconductor device includes a chip 1, a lead frame 3 mounted with the chip 1, a TAB tape 8 for electrically connecting the chip 1 and lead frame 3. The lead frame 3 is constituted of an alloy of Fe and 42Ni or the like, and formed integrally with a die pad 5, inner leads 7 and outer leads 6, as one component. The TAB tape 8 includes a plurality of leads 83 each having lead end portions 81 and 82, and a lead supporting portion 84 formed of resin film for forming these leads 83 integrally as one component. The chip 1 is bonded to the die pad 5 by, e.g., a conductive adhesive 12. The lead end portion 81 is connected to a bump electrode 11 formed on the major surface of the chip 1, while the lead end portion 82 is connected to the end of each inner lead 7 of the lead frame 3. The chip 1, TAB tape 8, inner leads 7 of the lead frame 3, and die pad 5 of the lead frame 3 are resin-sealed by a resin sealing member 2 such as epoxy resin. The die pad 5 is formed in the resin sealing member 2 and extends to a point several millimeters away from the periphery of the member 2. The die pad 5 has substantially the same circumference as that of the resin film 84 of the lead supporting portion. Furthermore, the die pad 5 is located below the inner leads 7 by the thickness of the chip 1.
In the semiconductor device illustrated in FIG. 2, the chip 1 generates heat during its operation, and the heat is radiated therefrom through the die pad 5 connected to the chip 1. Since the die pad 5 extends to the proximity of the periphery of the resin sealing member 2, the heat can be transmitted toward the proximity of the periphery of the member 2 through the die pad 5. For this reason, even though the power consumption of chip 1 is increased due to a high degree of integration and a high-speed operation, the chip 1 can radiate heat satisfactorily; thus the normal operation of the semiconductor device can be maintained.
If, however, the size of the die pad is set larger than that of the chip in order to improve in heat radiation, the fluid resin is hindered from flowing at the time of sealing. As described above, since the die pad 5 is located below the inner leads 7 by the thickness of the chip 1, the space enclosed with the TAB tape 8 and die pad 5 and including the chip 1, is very narrow. The resin sealing member 2 is obtained by arranging the lead frame, chip, and tape carrier (TAB tape) in a resin mold, pouring the fluid resin into the resin mold by a transfer mold, and hardening the fluid resin. The resin has to uniformly flow into the mold in order to uniformly form the resin sealing member. In the constitution shown in FIG. 2, however, the die pad 5 is increased in size, so that, as indicated by arrow 21, the resin is hindered from sufficiently flowing between the die pad 5 and TAB tape 8. As a result, an interval between the die pad and tape carrier becomes smaller than expected, and a short circuit or a crack is likely to occur in the resin sealing member itself because of its nonuniformity.